1. Field of the Invention
This invention relates broadly to surgical implants. More particularly, this invention relates to an anchoring portion of a stent particularly useful in synthetic grafts, although it is not limited thereto.
2. State of the Art
An aneurysm is an abnormal dilation of a layer or layers of an arterial wall, usually caused by a systemic collagen or structural defect. An abdominal aortic aneurysm (AAA) is an aneurysm in the abdominal portion of the aorta, usually located in or near one or both of the two iliac arteries or near the renal arteries. The aneurysm often arises in the infrarenal portion of the diseased aorta, for example, below the kidneys. A thoracic aortic aneurysm is an aneurysm in the thoracic portion of the aorta. When left untreated, the aneurysm may rupture, usually causing rapid fatal hemorrhaging.
Aneurysms may be classified or typed by their position as well as by the number of aneurysms in a cluster. Typically, abdominal aortic aneurysms may be classified into five types. A Type I aneurysm is a single dilation located between the renal arteries and the iliac arteries. Typically, in a Type I aneurysm, the aorta is healthy between the renal arteries and the aneurysm and between the aneurysm and the iliac arteries.
A Type II A aneurysm is a single dilation located between the renal arteries and the iliac arteries. In a Type II A aneurysm, the aorta is healthy between the renal arteries and the aneurysm, but not healthy between the aneurysm and the iliac arteries. In other words, the dilation extends to the aortic bifurcation. A Type II B aneurysm comprises three dilations. One dilation is located between the renal arteries and the iliac arteries. Like a Type II A aneurysm, the aorta is healthy between the aneurysm and the renal arteries, but not healthy between the aneurysm and the iliac arteries. The other two dilations are located in the iliac arteries between the aortic bifurcation and the bifurcations between the external iliacs and the internal iliacs. The iliac arteries are healthy between the iliac bifurcation and the aneurysms. A Type II C aneurysm also comprises three dilations. However, in a Type II C aneurysm, the dilations in the iliac arteries extend to the iliac bifurcation.
A Type III aneurysm is a single dilation located between the renal arteries and the iliac arteries. In a Type III aneurysm, the aorta is not healthy between the renal arteries and the aneurysm. In other words, the dilation extends to the renal arteries.
A ruptured abdominal aortic aneurysm is presently the thirteenth leading cause of death in the United States. The routine management of abdominal aortic aneurysms has been surgical bypass, with the placement of a graft in the involved or dilated segment. Although resection with a synthetic graft via a transperitoneal or retroperitoneal procedure has been the standard treatment, it is associated with significant risk. For example, complications include perioperative myocardial ischemia, renal failure, erectile impotence, intestinal ischemia, infection, lower limb ischemia, spinal cord injury with paralysis, aorta-enteric fistula, and death. Surgical treatment of abdominal aortic aneurysms is associated with an overall mortality rate of five percent in asymptomatic patients, sixteen to nineteen percent in symptomatic patients, and is as high as fifty percent in patients with ruptured abdominal aortic aneurysms.
Disadvantages associated with conventional surgery, in addition to the high mortality rate, include an extended recovery period associated with the large surgical incision and the opening of the abdominal cavity, difficulties in suturing the graft to the aorta, the loss of the existing thrombosis to support and reinforce the graft, the unsuitability of the surgery for many patients having abdominal aortic aneurysms, and the problems associated with performing the surgery on an emergency basis after the aneurysm has ruptured. Further, the typical recovery period is from one to two weeks in the hospital and a convalescence period, at home, ranging from two to three months or more, if complications ensue. Since many patients having abdominal aortic aneurysms have other chronic illnesses, such as heart, lung, liver and/or kidney disease, coupled with the fact that many of these patients are older, they are less than ideal candidates for surgery.
The occurrence of aneurysms is not confined to the abdominal region. While abdominal aortic aneurysms are generally the most common, aneurysms in other regions of the aorta or one of its branches are possible. For example, aneurysms may occur in the thoracic aorta. As is the case with abdominal aortic aneurysms, the widely accepted approach to treating an aneurysm in the thoracic aorta is surgical repair, involving replacing the aneurysmal segment with a prosthetic device. This surgery, as described above, is a major undertaking, with associated high risks and with significant mortality and morbidity.
Over the past five years, there has been a great deal of research directed at developing less invasive, endovascular, i.e., catheter directed, techniques for the treatment of aneurysms, specifically abdominal aortic aneurysms. This has been facilitated by the development of vascular stents, which can and have been used in conjunction with standard or thin-wall graft material in order to create a stent-graft or endograft. The potential advantages of less invasive treatments have included reduced surgical morbidity and mortality along with shorter hospital and intensive care unit stays.
Stent-grafts or endoprostheses are now Food and Drug Administration (FDA) approved and commercially available. Their delivery procedure typically involves advanced angiographic techniques performed through vascular accesses gained via surgical cut down of a remote artery, which may include the common femoral or brachial arteries. Over a guidewire, the appropriate size introducer will be placed. The catheter and guidewire are passed through the aneurysm. Through the introducer, the stent-graft will be advanced to the appropriate position. Typical deployment of the stent-graft device requires withdrawal of an outer sheath while maintaining the position of the stent-graft with an inner-stabilizing device. Most stent-grafts are self-expanding; however, an additional angioplasty procedure, e.g., balloon angioplasty, may be required to secure the position of the stent-graft. Following the placement of the stent-graft, standard angiographic views may be obtained.
Due to the large diameter of the above-described devices, typically greater than twenty French (3F=1 mm), arteriotomy closure typically requires open surgical repair. Some procedures may require additional surgical techniques, such as hypogastric artery embolization, vessel ligation, or surgical bypass in order to adequately treat the aneurysm or to maintain blood flow to both lower extremities. Likewise, some procedures will require additional advanced catheter directed techniques, such as angioplasty, stent placement and embolization, in order to successfully exclude the aneurysm and efficiently manage leaks.
While the above-described endoprostheses represent a significant improvement over conventional surgical techniques, there is a need to improve the endoprostheses, their method of use and their applicability to varied biological conditions. Accordingly, in order to provide a safe and effective alternate means for treating aneurysms, including abdominal aortic aneurysms and thoracic aortic aneurysms, a number of difficulties associated with currently known endoprostheses and their delivery systems must be overcome. One concern with the use of endoprostheses is the prevention of endo-leaks and the disruption of the normal fluid dynamics of the vasculature. Devices using any technology should preferably be simple to position and reposition as necessary, should preferably provide an acute, fluid tight seal, and should preferably be anchored to prevent migration without interfering with normal blood flow in both the aneurysmal vessel as well as branching vessels. In addition, devices using the technology should preferably be able to be anchored, sealed, and maintained in bifurcated vessels, tortuous vessels, highly angulated vessels, partially diseased vessels, calcified vessels, odd shaped vessels, short vessels, and long vessels. In order to accomplish this, the endoprostheses should preferably be highly durable, extendable and re-configurable while maintaining acute and long-term fluid tight seals and anchoring positions.
The endoprostheses should also preferably be able to be delivered percutaneously utilizing catheters, guidewires and other devices which substantially eliminate the need for open surgical intervention. Accordingly, the diameter of the endoprostheses in the catheter is an important factor. This is especially true for aneurysms in the larger vessels, such as the thoracic aorta. In addition, the endoprostheses should preferably be percutaneously delivered and deployed such that surgical cut down is unnecessary.
A typical percutaneously delivered endoprosthesis for treating an abdominal aortic aneurism is a bifurcated device having a main body and two legs. Ideally, the device lines the aorta and extends from just below the lowest renal artery into both iliac arteries up to the juncture with the hypogastric arteries. It is generally comprised of a fabric material with an outer metallic stent structure. The stent supports the graft and holds it open within the vessels.
The endoprosthesis is delivered to the aneurysm in the aorta by way of a delivery catheter. The delivery catheter containing the endoprosthesis is inserted through a small incision in the groin where it is threaded through the femoral artery and advanced to the location of the aneurysm. The surgeon uses fluoroscopy to guide the endoprosthesis and the endoprosthesis has several markers to help the surgeon visualize the graft during placement of the endoprosthesis. It is desirable that there be at least 10 mm of overlap between the endoprosthesis and a healthy vessel portion. Otherwise, an opening between the two can develop that can lead to leakage. Generally, visualization and surgical technique is good enough to permit the end of the endoprosthesis to be placed within two millimeters of the intended target site to ensure the desired overlap.
Once the endoprosthesis is secured within the abdominal aorta, blood flow can continue through the aorta, passing through the endoprosthesis without filling the aneurysm. This is intended to prevent further ballooning and possible rupture of the aneurysm.
The vis attached to the wall of the aorta by the outward pressure of the stent against the aortic wall and by barbs which extend outward from the stent and penetrate into the aortic wall. Referring to Prior Art FIG. 1, several axes can be defined with respect to a barb of a stent: an X-axis extending tangential to the circumference of the stent, a Y-axis extending radially outward from the circumference of the stent, and a Z-axis extending parallel to a central longitudinal axis of the stent and through the stent wall. In the prior art, a barb 910 of the stent 912 is generally a longitudinally bent portion at the end of the stent which is coupled at 914 so as to spring outward relative to the longitudinal axis of the stent by sweeping an angle about the X-axis. The strength of the barb is dependent on the geometry of the coupling point 914. However, such coupling point 914 stiffness is compromised to permit the barb 910 to spring open to the desired angle once the end of the stent is released from the delivery catheter due to strain limitations. Referring to Prior Art FIG. 2, this weakness allows the barb to be deformed from a naturally open angle of α1 (e.g., approximately) 45° to a more perpendicular orientation of angle α1+α2 (e.g., approximately 90°) when the stent is subject to the forces F of blood flow. Thus, the manner in which the barb opens compromises the strength of the barb. Furthermore, the deformed configuration of such barbs may create stress risers and relatively high strain in the barb structure which results in barb fatigue.
In addition, referring to Prior Art FIG. 2, the angle α1 at which the barb 910 naturally initially opens to engage tissue is typically acute. Thus, to fully engage the aortic tissue with no further movement, the tissue of the aortic wall must move along the barb until seated against the outer surface of the stent. Such relative movement between the tissue and barb can cause the endoprosthesis to migrate relative to the aortic wall. Even migration of a few millimeters may result in undesirable leakage if there is less than desirable endoprosthesis/vessel overlap.